The equilibrium moisture content, adsorption and desorption rate of the clay raw soil materials and raw soil materials have been tested experimentally. The humidity-controlling mechanism of the raw soil material is investigated based on the micro-analysis of the scanning electron microscope and the absorption/desorption theory of materials. The results show that though the density of the raw soil materials is higher than those of the clay raw materials, the raw soil materials have the similar microstructure like the clay raw materials. Therefore, the raw soil materials have a good humidity-controlling performance. The maximum equilibrium moisture content of the raw soil material which is made from 75% pure soil material and 25% modified admixture is 11.59%. The absorption and desorption rate can reach 0.030 4 Kg/Kg·d and 0.025 7Kg/Kg·d respectively. The humidity controlling mechanism of the raw soil material is that its cellular microstructure has the effects of capillary channel, chemical adsorption and surface physical adsorption on water vapor, in which the influence of the capillary channel and chemical adsorption on the adsorption effect of water vapor are strong while the surface physical adsorption effect is weak. Therefore, the raw soil material is able to control the indoor air relative humidity automatically.